Direct access storage devices (DASD) have become part of every day life, and as such, expectations and demands continually increase for greater speed for manipulating data and for holding larger amounts of data. To meet these demands for increased performance, the mechanical assembly in a DASD device, specifically the Hard Disk Drive (HDD) has undergone many changes.
Market demand for more performance from the HDD has led to advances in motor-hub assembly technology. Of particular interest is the introduction of fluid dynamic bearings (FDB). By using an FDB in an HDD, a disk can be spun at faster speeds with less unwanted vibrations traditionally experienced with ball bearings. Since there is minimal contact between moving parts, an FDB will not wear as quickly as a traditional ball bearing.
A fluid dynamic bearing is the result of a thin layer of fluid, such as oil, moving between two juxtaposed surfaces and thus creating a barrier between the two surfaces that prevents their contact. Methods have been varied for moving the fluid to generate the FDB. One early method of moving the fluid was to use a pump that was external to the juxtaposed surfaces. This is costly for HDD application. The more widely used method for moving the fluid is to create a pattern of grooves on the juxtaposed surfaces that create internal pressure on the fluid when such surfaces are moved with respect to each other. To accommodate the occasional contact of the two juxtaposed surfaces, they may be given a protective coating that is resistant to wear. With recent advances in FDB design, wear of the juxtaposed surfaces has been minimized, and the wear resistant coating can now be eliminated.
Normally, the materials used to fabricate the two juxtaposed surfaces in an FDB are chosen for their ease of machinability while meeting the physical requirements of an FDB. The preferred material is categorized as yellow metal. Yellow metal is any metal that contains copper as its major element.
One deleterious effect of operating an FDB with exposed yellow metal surfaces is that the fluid in the FDB reacts with the yellow metal. For example, the thin layer of moving fluid can corrode the yellow metal causing the performance of the FDB to deteriorate at an unsatisfactory rate. One solution to the problem is to reintroduce a wear resistant coating such as a thin layer of nickel-phosphorous to the yellow metal. The application process typically used to apply nickel-phosphorous is an electro-plating process, which is widely known and understood in the industry.
However, the electro-plating process and additional manufacturing steps have associated costs that increase the overall cost of the FDB.